Automotive vehicles are commonly equipped with a radio and a corresponding antenna system to receive amplitude modulation (AM) and frequency modulation (FM) broadcast radio signals. One conventional vehicle antenna system includes a mast antenna vertically extending from the body of the vehicle. Mast antennas are generally limited in signal performance, add wind noise and drag to the vehicle, and are susceptible to corrosion and damage.
Another conventional vehicle radio antenna includes a backlite antenna system in which antenna elements are embedded in a rear window of the vehicle. Examples of backlite antenna systems are disclosed in U.S. Pat. Nos. 5,610,619, 5,790,079, and 5,099,250. The vehicle antenna set forth in U.S. Pat. No. 5,099,250 utilizes the defogger elements encapsulated in the back window of the vehicle as antenna elements to receive broadcast radio signals. Conventional antenna systems that integrate the antenna with the defogger heating elements in the rear window of a vehicle typically require bifilar or toroidal chokes coupled between the conductive window elements and the vehicle DC power supply to separate the received antenna signal from the high current signals that heat the defogger elements. These chokes provide low impedance paths for the propagation of large current flow necessary to power the defogger heating elements, and high impedance paths against the propagation of the radio signals. The chokes are generally incorporated in an antenna impedance matching network in order to match the output of the antenna elements to the input of the amplifier associated with the vehicle radio to reduce the attenuation of power transfer from the antenna elements to the radio.
Typical impedance matching networks are specially designed and vary from one type of vehicle to other types of vehicles to realize the greatest efficiency in impedance matching. This is generally because the capacitance created between the conductive elements in the vehicle body varies from vehicle to vehicle. Further, prior art antenna grid patterns are often directional at FM frequencies, and have relatively low gain at AM frequencies.
It is therefore desirable to provide for an antenna system that employs the defogger heating elements in a vehicle window, such as the rear window, and includes an antenna impedance matching network that can be incorporated into a variety of vehicles. It is further desirable to provide for such an antenna system that does not require lumped matching components.